We first need to identify what a superlative form of an adjective is. That is when the adjective is put in a comparison, and is done so in it's higher form. For example, take the adjective "great". The superlative form would be "greatest". That would imply that there are, including the object that the superlative adjective is describing, 3 or more things. So, you use it only when comparing 3 or more nouns.